Wireless communication networks are increasingly being used for wireless communications with various types of wireless user equipment. The wireless network itself may include a plurality of space-apart wireless base stations, also commonly referred to as “base stations”, “radio access nodes”, “RAN nodes”, “NodeBs”, “eNodeBs” or simply as “nodes”, that define a plurality of cells, and a core network that controls the base stations and interfaces the base stations with other wired and/or wireless networks. The nodes may be terrestrial and/or space-based. The nodes communicate with wireless User Equipment (UE), also referred to as “user equipment”, “wireless terminals” or “mobile stations”, using radio resources that are allocated to the wireless network. The radio resources may be defined in terms of time (for example, in a Time Division Multiple Access (TDMA) system), frequency (for example, in a Frequency Division Multiple Access (FDMA) system) and/or code (for example, in a Code Division Multiple Access (CDMA) system). The nodes may use licensed and/or unlicensed frequency spectrum. Radio resources may be assigned to UEs by the wireless network upon initial communication and may be reassigned due to, for example, movement of the UEs, changing bandwidth requirements, changing network traffic, etc.
In many existing wireless cellular communication systems and methods, pilot symbols are transmitted for each antenna or antenna port over radio resource elements that are non-overlapping in time and in frequency with those pilot symbols transmitted for other antennas or antenna ports. For example, in the current release (Rel-10) of Long Term Evolution (LTE) wireless technology, known Reference Symbols (RSs) or pilot symbols are transmitted at various time instants and frequencies for different antenna ports. Using these known RSs, a receiver can estimate the channel response from each transmit antenna to each receive antenna across various times and frequencies.
Moreover, in many existing wireless cellular systems, synchronization signals are also transmitted by nodes in order to allow a UE to find and acquire synchronization to a cell within a radio access network. For example, in the current release (Rel. 10) of LTE, two special signals are transmitted on the LTE downlink: a Primary Synchronization Signal (PSS) and a Secondary Synchronization Signal (SSS). The PSS and SSS may have similar structure, but the time-domain positions of the synchronization signals within a frame may differ somewhat depending on whether the cell is operating in Frequency Division Duplex (FDD) or Time Division Duplex (TDD) mode. In the case of FDD, the PSS is transmitted within the last symbol of the first slot of subframes 0 and 5, while the SSS is transmitted in the second last symbol of the same slot (i.e., just prior to the PSS). In contrast, in the case of TDD, the PSS is transmitted within the third symbol of subframes 1 and 6, while the SSS is transmitted in the last symbol of subframes 0 and 5 (i.e., three symbols ahead of the PSS). Once the UE has detected and identified the PSS of the cell, it has found the timing of the cell and the position of the SSS, which has a fixed offset relative to the PSS, along with the cell identity within a single identity group. From the SSS, the UE may find the frame timing and the cell identity group. The PSS and SSS are collectively referred to herein as “synchronization signals”.
LTE allows the aggregation of multiple carriers to send signals between a node and user equipment. The aggregation of carriers in this manner is shown pictorially in FIG. 1. The main carrier is referred to as a primary carrier while the additional carrier is referred to as a secondary carrier. In Rel-10 of LTE, the secondary carrier is generally required to have the features of a regular LTE carrier. That is, it carries synchronization signals for time and frequency synchronization, reference symbols for channel estimation and control signals for data allocations and other control functions much like the primary carrier.
The reuse of carrier frequencies across multiple sites within a cellular network may give rise to interference issues. Additionally, synchronization and reference signals transmitted by nodes in the network may also be wasteful of energy. Interference issues may be especially problematic in so-called “heterogeneous networks” where carrier aggregation is accomplished using micro or pico base stations.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to claims in this application and any application claiming priority from this application, and are not admitted to be prior art by inclusion in this section.